1. Field of the Invention
The present invention relates to an electrophotographic developing apparatus of a type employed in an electrophotographic system utilizing a so-called single-component developing material, which apparatus is operable to develop an electrostatic latent image formed on a photosensitive medium into a toner image.
2. Description of the Prior Art
The electrophotographic developing apparatus of the type to which the present invention pertains has come to be largely employed in an electrophotographic copying machine, a laser beam printer, a facsimile machine or the like. A developing method employed in the electrophotographic developing apparatus is well known in the art and is disclosed in, for example, U.S. Pat. No. 4,903,634.
The prior art electrophotographic developing apparatus will now be discussed with reference to FIG. 4 which illustrates, in a schematic side sectional representation, the structure of the prior art developing apparatus.
In FIG. 4, reference numeral 21 represents a photoreceptor drum; reference numeral 22 represents a developing roller disposed in face-to-face relation with the photoreceptor drum 21 for carrying toner which is a developing material; reference numeral 23 represents a supply roller; reference numeral 24 represents a charged layer regulating blade; reference numeral 25 represents a hopper accommodating therein a mass of one-component (i.e. single-component) toner material; reference numeral 26 represents a paddle board; and reference numeral 27 represents a direct current power source.
The prior art developing apparatus of the above described construction operates in the following manner. The toner material within the hopper 25 is delivered by the paddle board 26 onto the supply roller 23 which subsequently delivers the toner material onto the charge transfer roller 22 to form a layer of toner material on the charge transfer roller 22. This toner layer is, during the continued rotation of the charge transfer roller 22 in the direction shown by the arrow, regulated by the charged layer regulating blade 24 to thereby regulate the amount of toner deposited on the charge transfer roller 22 and at the same time to form the toner layer which has been triboelectrically charged. This toner layer is subsequently deposited faithfully on an electrostatic latent image on the photoreceptor drum 21 by the effect of an electric field developed by the direct current power source 27.
A developing process practiced with the use of the one-component toner material is available in two systems; a contact developing system and a non-contact developing system. In the contact developing system, the developing roller 22 is made of rubber material having an elasticity and is held in contact under pressure with the photoreceptor drum 21 through a toner layer. (See the Japanese Laid-open Patent Publications No. 1-310303 and No. 2-1881.) On the other hand, in the non-contact developing system, the developing roller 22 has a toner layer deposited thereon and is spaced a distance from the photoreceptor drum 21 thereby defining a gap between the developing roller 22 and the photoreceptor drum 21. (See the Japanese Laid-open Patent Publication No. 62-89975.)
However, the prior art developing apparatus of the above described construction has the following problems.
In the first place, the developing member is required to have a number of properties and, therefore, the freedom of choice of material is limited. By way of example, those properties include:
(1) Electroconductivity with which charge can be imparted successively to the toner material, PA1 (2) An insulating property with which, even though a defect is found on the photoreceptor member, a leak can be prevented by a developing bias, PA1 (3) Resistance to friction brought about by the charged layer regulating blade, PA1 (4) Surface roughness by which a predetermined amount of toner material and a predetermined amount of toner charge can be available, PA1 (5) Charge imparting power by which the toner material can be charged to a required polarity, PA1 (6) A high releasing property by which the toner material can readily be released in order to accomplish a faithful deposit of the toner material on a delicate electrostatic latent image, PA1 (7) High shape machinability, and PA1 (8) High stability of those properties relative to a change in environment.
As discussed above, the developing roller must satisfy those characteristic requirements including the electroconductivity and the insulating property which are in compatible with each other. In order to satisfy all of those properties, there is no way other than to employ material in the vicinity of limits of those properties and it is indeed difficult for a single member such as the developing roller to satisfy all of those conditions.
Specifically, since in the contact development the developing member is held in contact with the latent image carrier, the developing member must necessarily be an elastic member. For this reason, an inexpensive metallic roller excellent in surface roughness, shape machinability, friction resistance, electroconductivity and charge imparting power cannot be employed in a toner charging and layer regulating section. In general, the elastic roller necessary in the contact developing system has a poor toner releasing property. According to an experiment during which polyurethane rubber imparted with an electroconductivity was employed for the developing roller, adherence and embedding of toner to the developing roller occurred. Because of this, a developing toner amount could not be secured, resulting in toner charge amount as a result of a change in surface condition.
In the second place, the amount of toner per unit surface area of the developing roller confronting the photoreceptor drum (hereinafter referred to as a toner amount) and the amount of charge of toner per unit weight (hereinafter referred to as a toner charge amount) cannot be chosen as desired. Where the toner charge amount is small, an image quality such as a resolution will be adversely affected, but when the toner amount is small, an image density will be reduced.
Where the one-component toner material containing no carrier is processed by the blade to form a toner layer and is at the same time charged electrically, a correlation is found between the toner amount and the toner charge amount. The relationship between the toner amount and the toner charge amount on the developing roller when the surface roughness of the developing roller is changed is shown in FIG. 5. A curve shown by the solid line in FIG. 5 is determined by the coefficient of friction and the charge imparting power of material for the toner material and the charge member (developing roller and toner layer regulating blade) attributable to a charge system. When respective material for the developing roller, the toner material and the charge layer regulating blade has been determined and when the surface roughness of the developing roller is changed, the following relationship is established: EQU if w1&gt;w2, then q1&lt;q2,
wherein q2 (point A) represents the toner charge amount when the toner amount is w2, and q1 (point B) represents the toner charge amount when the toner amount is w1.
Accordingly, in order to concurrently achieve the sufficient toner amount W1 required for the development and the sufficient toner charge amount q2, material excellent in charge imparting power to the toner material is required for each of the various members. However, according to the prior art, it has been difficult to find an ideal material effective to satisfy the relationship between the required toner amount and the required charge amount such as shown by a curve shown by the broken line.
In the case of the contact development, wear of the photoreceptor drum comes to be a problem. Since the toner amount becomes small as discussed above when the toner charge amount is high, an attempt has been made to rotate the developing roller at a higher speed than the photoreceptor drum to secure the toner amount on the photoreceptor drum such as disclosed in, for example, the Japanese Laid-open Patent Publication No. 1-310302. In such case, wear of the photoreceptor drum comes to be a problem. In particular, when the process speed is high, the photoreceptor drum tends to wear rapidly, resulting in a considerably reduced lifetime of the photoreceptor drum.
In the case of the non-contact development, an edge effect in which a relatively large amount of toner deposits at an edge portion of the electrostatic latent image tends to occur when a difference in speed exists between the developing member and the photoreceptor member. For this reason, no difference in speed can be imparted between the developing roller and the photoreceptor drum. Accordingly, it is necessary to secure the toner amount sufficient to accomplish a density on the developing roller. However, when the sufficient toner amount is desired, no sufficient toner charge amount can be satisfied, resulting in an image of low resolution.